Bug 1891262 - Update Flatpak manifest to align with Flathub's new brand guidelines. r=rjl

Differential Revision: https://phabricator.services.mozilla.com/D207349

--HG--
extra : amend_source : 34c0915ebad312ff14f52a4cdd65df259c65698d

taskcluster/docker/tb-flatpak/org.mozilla.Thunderbird.appdata.xml.in

@@ -59,6 +59,11 @@
     <mediatype>x-scheme-handler/mid</mediatype>
   </provides>
 
+  <branding>
+    <color type="primary" scheme_preference="light">#99c1f1</color>
+    <color type="primary" scheme_preference="dark">#043571</color>
+  </branding>
+
   <content_rating type="oars-1.1" />
   <url type="homepage">https://www.thunderbird.net/</url>
   <url type="bugtracker">https://bugzilla.mozilla.org/</url>